In recent years, as the demand for indoor voice communication and data communication has grown due to the spread of mobile phones, the development of a home-use base station installed indoors has been pursued. Since a range covered by a home-use base station is considerably smaller than that of a base station installed outdoor, the range is called “femtocell”. Accordingly, a home-use base station is referred to as “femto base station” hereinafter.
In base stations in existing mobile communication networks, the radio frequency (hereinafter, simply called “RF”) used for communication with a mobile station in each cell formed by a base station is set in advance to a fixed value. In contrast to this, for femto base stations, a way of autonomously performing radio resource control has been studied. For example, in a femto station, the RF used for communication with a mobile station is not determined in advance. Therefore, a way of selecting an RF in which the signal reception level of each selection candidate for the RF to be used is measured and then the RF having the minimum reception level is autonomously selected for the communication with a mobile station has been studied. Patent document 1 (page 14, line 8 to page 15, line 10) discloses a method like this.
Its specific example is explained with reference to FIG. 14. Referring to FIG. 14, a base station 811 communicates with a mobile station (not shown) by using radio frequencies RF1 and RF2, while a base station 812 communicates with a mobile station by using a radio frequency RF1. In such a case, if the radio frequencies RF1 and RF2 are selection candidates, a femto base station 821 measures the signal reception level of each of the radio frequencies RF1 and RF2. Then, the femto base station 821, which is located within the cell of the base station 812, selects and uses RF2 because the reception level of RF2, which is not used by the base station 812, is lower than that of RF1, which is used by the base station 812.
The femto base station like the one described above has been studied for use in systems such as WCDMA and E-UTRAN. In WCDMA, data transmission is performed by using an dedicated channel, of which transmission power is controlled, on an uplink and a downlink, or is performed by using a shared channel on a downlink as shown in Non-patent document 1. Further, in E-UTRAN, a radio frequency band is divided into a plurality of PRBs (Physical Resource Blocks) as shown in Non-patent document 2. A scheduler provided in an E-UTRAN base station assigns PRBs, and a base station performs data transmission with a mobile station by using an assigned PRB.
[Patent Document 1]
UK Patent Application Publication No. 2428937 A
[Non Patent Document 1]
3GPP TS 25.214 V7.3.0 (2006-12), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Physical layer procedures (FDD) (Release 7)
[Non Patent Document 2]
3GPP TS 36.300 V8.1.0 (2007-06), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2 (Release 8)